An important role for inflammatory cytokines in rheumatoid arthritis (RA) is well established and has been validated by successful therapeutic targeting of tumor necrosis factor (TNF). TNF is a key pathogenic factor in approximately two thirds of RA patients who are responsive to TNF blockade therapy, and in many additional autoimmune/inflammatory diseases. Mechanisms by which TNF drives RA pathogenesis include activation of synovial cells to produce factors that drive inflammation and tissue destruction. Molecular mechanisms by which TNF acutely activates cells and promotes an acute inflammatory response have been extensively studied and are well understood. Surprisingly little is known about the effects of chronic exposure to TNF, as is relevant for RA pathogenesis, and about mechanisms by which TNF activates cells in the setting of chronic inflammation. It is important to understand mechanisms of chronic TNF action to gain additional insights into the functions of TNF in chronic inflammation and to discover new therapeutic targets and approaches to therapy based on inhibiting selective TNF functions that are important for chronic inflammation (and thus potentially sparing acute functions important for host defense). Therefore, we have initiated experiments using primary human monocytes/macrophages and RA synovial macrophages to explore the effects of chronic TNF exposure and underlying molecular mechanisms. We have found that in addition to inducing a well characterized acute inflammatory response, longer term TNF exposure induces high STAT1 expression and an IFN response, and attenuates signaling by homeostatic cytokines such as IL-10 and IL-27. The human disease relevance of these findings has been validated by observing similar IFN/STAT1 responses and resistance to IL-10 and IL-27 in macrophages obtained from clinical samples obtained from inflamed joints of RA patients. We postulate that these newly discovered TNF functions contribute to its pro-inflammatory properties and role in RA pathogenesis by attenuating homeostatic responses and priming macrophages for STAT1-mediated responses. Furthermore, we propose that reversal of IL-10 and IL-27 resistance (either by pharmacologically augmenting their signaling pathways or by removing inducers of resistance) may represent a fruitful therapeutic approach. In this application, we will investigate mechanisms by which TNF antagonizes homeostatic cytokines, and the functional significance of this inhibition. We will use primary human cells and RA synovial macrophages to maximize the relevance of results for human RA pathogenesis, and will also utilize in vivo murine models of TNF-driven arthritis and inflammation. We propose that our studies will lead to a new view of TNF biology and how TNF can contribute to RA pathogenesis. These studies will potentially yield insights into mechanisms of TNF action that can be therapeutically targeted while preserving acute functions of TNF that are important in host defense.